1. Field of the Invention
The present invention relates to a dual optical pick-up head that is capable of accessing both CD (compact disc) disc and DVD (digital versatile disc) discs. More particularly, the present invention relates to a dual optical pick-up head that not only employs two objective lenses and one actuator but also rids the requirement to switch position of objective lenses while different discs are detected.
2. Description of the Prior Art
The DVD is an optical disc that is capable of recording a large amount of digital information. Its recording scheme allows the recording of digital information, such as video, computer information or the like on a disc having the same diameter as the CD, at a recording density of six to eight times as high as the CD. No matter how, although the storage amount of DVD disc is larger than CD disc, CD disc is more widespread used in many fields than DVD disc. Thus, it is common commercial benefit that a DVD player uses a dual optical pick-up head not only work with DVD discs but also compatibly work with traditional CD discs.
Evidently, when picking up data from a DVD disc the dual optical pick-up head works with a shorter focal length of objective lens and a shorter wavelength laser beam. Moreover, when picking up data from a CD disc the dual optical pick-up head works with a larger focal lengths and a larger wavelength laser beam. Therefore, the dual optical pick-up head should be capable to work with different wavelength lasers and different focal-length objective lenses.
In order to solve such a problem, one conventional method uses two objective lenses for both DVD and CD separated where two lenses are switched in accordance with the type of disc. Another conventional method uses a correcting lens that is inserted into a collimator portion, thereby correcting the aberration due to the disc. The other conventional method uses a two-focal objective lens such as holographic optical element (HOE) as an objective lens. A further conventional method uses a liquid crystal (LCD) shutter to modulate laser beam. Among these conventional methods, two methods are more popular than others and are explained in posterior paragraphs.
FIG. 1A and FIG. 1B show the mechanism of the dual optical pick-up head with the usage of the two-focal point objective lens. When CD disc 10 is accessed and 780 nm wavelength laser beam 11 is incident, as shown in FIG. 1A, laser beam 11 transmits through two-focal point objective lens 12 and is focused on CD disc 10. Similarly, when DVD disc 13 is accessed and 650 nm wavelength laser beam 14 is incident, as shown in FIG. 1B, laser beam 14 transmits through two-focal point objective lens 12 and is focused on DVD disc 13. Obviously, the optical performance of the two-focal point objective lens 12 is complicated, and then the structure is complex and the cost is expensive.
FIG. 2A and FIG. 2B show the mechanism of the dual optical pick-up head with the usage two objective lenses. When CD disc 23 is accessed and 780 nm wavelength laser beam 24 is incident, as shown in FIG. 2A, switching machine 22 puts CD objective lens 20 into path of 780 nm wavelength laser beam 24 and then 780 nm laser wavelength beam 24 is focused on CD disc 23. Similarly, when DVD disc 26 is accessed and 650 nm wavelength laser beam 25 is incident, as shown in FIG. 2B, switching machine 22 put DVD objective lens 21 into path of 650 nm wavelength laser beam 25 and then 650 nm laser beam 25 is focused on DVD disc 26. Evidently, the path of 780 nm wavelength laser beam 24 is equal to the path of 650 nm wavelength laser beam 25. Therefore, the positions of both CD objective lenses 20 and DVD objective lens 21 are switched to put required objective lens into the path of laser beam 24/25 while different discs 23/26 is detected. Clearly, the case requires complicated switching machine 22 that occupies a large space is necessary to switch objective lens, and then it is not suitable for a compact dual optical pick-up head.
Besides, in the method of using a LCD shutter, the polarization of the laser beam and location of LCD must be properly arranged, then the design is difficult to actualize. Furthermore, the LCD needs additional driving circuit to maintain shutter effect.
According to previous discussion, although several conventional ways are presented to compass the subject, none of them efficient and compact enough. Therefore, it is necessary to develop an economical dual optical pick-up head for accessing both CD and DVD discs.
One main object of the invention is to propose a dual optical pick-up head that can detect DVD discs and CD discs with one actuator and two objective lenses, especially to propose a dual optical pick-up head without the necessarily of a switching machine to switch these objective lenses while different discs are detected.
In comparison with conventional dual optical pick-up head with two objective lenses, a significant character of the proposed dual optical pick-up head is that these objective lenses do not have to switch their position while different discs are detected.
Therefore, the position that laser beam project into disc does not have to be located in a radius of disc that is parallel to the guide rod. Hence, the location of these two objective lenses is elastic and the configuration of the proposed dual optical pick-up head is convertible. Besides, because there is no requirement to use two-focal objective lens then the structure and fabrication of the proposed dual optical pick-up head is simplified and compact.